WO2017111510A1

WO2017111510A1 - Non-magnetic steel material having excellent hot workability and manufacturing method therefor

Info

Publication number: WO2017111510A1
Application number: PCT/KR2016/015121
Authority: WO
Inventors: 이운해; 김성규; 이순기; 김용진; 오홍열
Original assignee: 주식회사 포스코
Priority date: 2015-12-23
Filing date: 2016-12-23
Publication date: 2017-06-29

Abstract

Provided according to one embodiment of the present invention are a non-magnetic steel material and a method for manufacturing the same. The steel material comprises 15-27 wt% of manganese (Mn), 0.1-1.1 wt% of carbon (C), 0.05-0.50 wt% of silicon(Si),0.03 wt% or less (0% exclusive) of phosphorus (P),0.01 wt% or less (0% exclusive) of sulfur (S), 0.050 wt% or less (0% exclusive) of aluminum (Al), 5 wt% or less (0% inclusive) of chromium (Cr), 0.01 wt% or less (0% inclusive) of boron (B), 0.1 wt% or less (0% exclusive) of nitrogen (N), and a balance amount of Fe and inevitable impurities, has an index of sensitivity of 3.4 or less, the index of sensitivity being represented by the following relational expression (1): [Relational expression 1] -0.451+34.131*P+111.152*Al-799.483*B+0.526*Cr≤3.4 (wherein [P], [Al], [B] and [Cr] each mean a wt % of corresponding elements), and contains a microstructure with austenite at an area fraction of 95 % or greater therein.

Description

Non-magnetic steel with excellent hot workability and manufacturing method

The present invention relates to a nonmagnetic steel having excellent hot workability and a method of manufacturing the same.

Transformer structures include enclosures, lock plates, and the like, and the steel used therein requires excellent nonmagnetic properties.

Recently, non-magnetic steels such as chromium (Cr) and nickel (Ni) are completely excluded. Instead of a large amount of manganese (Mn) and carbon (C), austenite is stabilized by adding a large amount of nonmagnetic properties. have. The austenitic phase is paramagnetic and has a low permeability and superior nonmagnetic properties to ferrite.

A high manganese (Mn) steel having austenite containing a large amount of carbon is characterized by high austenite phase stability, and thus is suitable for use as a nonmagnetic steel.

However, when aluminum (Al), phosphorus (P) and the like are contained in austenite in a large amount of residual elements generated in manufacturing high manganese steel, the crack sensitivity of the steel is improved at high temperatures. This is due to the low hot ductility and internal grain boundary oxidation at high temperatures, and the high crack sensitivity of the steel has a great influence on the surface quality of the steel at room temperature.

Therefore, it is necessary to develop a nonmagnetic steel having excellent surface quality while reducing cracking sensitivity of the steel.

One preferred aspect of the present invention is to provide a non-magnetic steel having excellent hot workability with low hot crack sensitivity and excellent surface quality.

Another preferred aspect of the present invention is to provide a method for producing a non-magnetic steel having excellent hot workability with low hot crack sensitivity and excellent surface quality.

According to a preferred aspect of the present invention, manganese (Mn): 15-27% by weight, carbon (C): 0.1-1.1% by weight, silicon (Si): 0.05-0.50% by weight, phosphorus (P): 0.03% by weight (Excluding 0%), sulfur (S): 0.01% by weight (excluding 0%), aluminum (Al): 0.050% by weight (excluding 0%), chromium (Cr): 5% by weight (including 0%) ), Boron (B): 0.01% by weight or less (including 0%), nitrogen (N): 0.1% by weight or less (excluding 0%), the balance Fe and other unavoidable impurities, and is represented by the following relation (1) The sensitivity component index value is 3.4 or less,

[Relationship 1]

-0.451 + 34.131 * P + 111.152 * Al-799.483 * B + 0.526 * Cr≤3.4

(The above [P], [Al], [B] and [Cr] mean each weight percent of the corresponding element)

There is provided a non-magnetic steel having excellent hot workability in which the microstructure comprises 95% or more of austenite in an area fraction.

The average grain size of the austenite may be 10 μm or more.

According to another preferred aspect of the present invention, manganese (Mn): 15 to 27% by weight, carbon (C): 0.1 to 1.1% by weight, silicon (Si): 0.05 to 0.50% by weight, phosphorus (P): 0.03 weight % Or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), aluminum (Al): 0.050% or less (excluding 0%), chromium (Cr): 5% or less (0%) Boron (B): 0.01 wt% or less (including 0%), nitrogen (N): 0.1 wt% or less (excluding 0%), balance Fe and other unavoidable impurities, and are represented by the following relational formula (1) Preparing a slab having a sensitivity component index value of 3.4 or less;

[Relationship 1]

-0.451 + 34.131 * P + 111.152 * Al-799.483 * B + 0.526 * Cr≤3.4

A slab reheating step of reheating the slab at a temperature of 1050-1250 ° C .;

A hot rolling step of hot rolling the reheated slab to obtain a hot rolled steel; And

Provided is a method of manufacturing a non-magnetic steel having excellent hot workability, including a cooling step of cooling a hot rolled steel.

According to one embodiment of the present invention, it is possible to provide a nonmagnetic steel having a uniform austenite phase, excellent nonmagnetic properties, low crack sensitivity, and good surface quality, and a method of manufacturing the same.

Figure 1 shows the degree of surface quality for measuring the crack sensitivity as a score, the rating 1 is a crack does not occur on the surface, the rating 1.5 is a state with a fine flaw, the rating 2 is a crack propagation caused a large crack Status is displayed.

2 is an example of a schematic diagram for explaining a crack sensitivity measurement site for crack sensitivity evaluation.

3 is a graph showing the relationship between crack sensitivity and sensitivity component index values.

Hereinafter, preferred embodiments of the present invention will be described.

However, embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

In addition, embodiment of this invention can be modified in various other forms, The range of this invention is not limited to embodiment described below.

In addition, the inclusion of any component throughout the specification means that it may further include other components, except to exclude other components unless specifically stated otherwise.

EMBODIMENT OF THE INVENTION Hereinafter, the nonmagnetic steel material excellent in hot workability by this invention is demonstrated in detail.

Non-magnetic steel having excellent hot workability according to a preferred aspect of the present invention is manganese (Mn): 15 to 27% by weight, carbon (C): 0.1 to 1.1% by weight, silicon (Si): 0.05 to 0.50% by weight, phosphorus (P): 0.03% or less (excluding 0%), Sulfur (S): 0.01% or less (excluding 0%), Aluminum (Al): 0.050% or less (excluding 0%), Chromium (Cr): 5 Below 0% by weight (including 0%), boron (B): below 0.01% by weight (including 0%), nitrogen (N): below 0.1% by weight (excluding 0%), balance Fe and other unavoidable impurities, The sensitivity component index value expressed by the relation (1) is 3.4 or less, and has a microstructure containing 95% or more of austenite as an area fraction.

[Relationship 1]

-0.451 + 34.131 * P + 111.152 * Al-799.483 * B + 0.526 * Cr≤3.4

First, the component and component range of steel materials are demonstrated.

manganese( Mn ): 15-27 wt%

The content of the manganese is preferably limited to 15 to 27% by weight.

The manganese is an element that serves to stabilize austenite.

The manganese may be included in an amount of 15 wt% or more to stabilize the austenite phase at cryogenic temperatures.

When the content of manganese is less than 15%, in the case of steel having a low carbon content, metastable epsilon (ε) -martensite is formed and is easily transformed into alpha prime (α ′)-martensite by processing organic transformation at cryogenic temperatures. The toughness of the steel can be lowered.

In addition, in the case of steels in which the carbon content is increased in order to secure the toughness of the steel, physical properties of the steel may be drastically reduced due to carbide precipitation.

When the content of the manganese exceeds 27% by weight, the economical efficiency of the steel may be reduced due to the increase in manufacturing cost.

More preferred manganese content is 15-25% by weight, even more preferred manganese content is 17-25% by weight.

Carbon (C): 0.1-1.1 wt%

The content of the carbon is preferably limited to 0.1 to 1.1% by weight.

The carbon is an element that stabilizes austenite and increases the strength of steel.

The carbon may serve to lower Ms and Md which are transformation points of austenite, epsilon (ε) -martensite or alpha prime (α ′)-martensite by a cooling process or processing.

If the carbon content is less than 0.1% by weight, the stability of austenite is insufficient to obtain austenite that is stable at cryogenic temperatures, and is easily absorbed by external stress such as epsilon (ε) -martensite or alpha prime (α ')-martensine. Machining organic transformation to the site can reduce the toughness and strength of the steel.

When the content of the carbon exceeds 1.1% by weight, the toughness of the steel can be rapidly deteriorated due to carbide precipitation, the strength of the steel is too high, the workability of the steel can be reduced.

More preferred carbon content is 0.1 to 1.0% by weight, even more preferred carbon content is 0.1 to 0.8% by weight.

Si : 0.05 ~ 0.5 wt%

Si is an element which is indispensably added as a deoxidizer like Al. When Si is excessively added, oxides are formed at grain boundaries to reduce high-temperature ductility, causing cracks and the like, which may lower the surface quality. However, since excessive cost is required to reduce the amount of Si added in the steel, the lower limit thereof is preferably limited to 0.05%. Since the oxidizing property is higher than Al, when it is added in excess of 0.5%, the oxide is formed to form cracks, so the surface quality is lowered. Therefore, the Si content is preferably limited to 0.05 to 0.5%.

chrome( Cr ): 5% or less (including 0%)

Chromium stabilizes austenite up to the range of an appropriate amount of addition, thereby improving impact toughness at low temperatures, and solid-solution in austenite increases the strength of steel. Chromium is also an element that improves the corrosion resistance of steels. However, chromium is a carbide element, in particular, an element that reduces carbide impact by forming carbide at the austenite grain boundary. Therefore, the content of chromium is preferably determined in consideration of the relationship with carbon and other elements added together, and in view of being an expensive element, the content is preferably limited to 5% by weight or less.

More preferred chromium content is 0 to 4% by weight and even more preferred chromium content is 0.001 to 4% by weight.

Boron (B): 0.01% by weight or less (including 0%)

The content of boron is preferably limited to 0.01% by weight or less.

Boron is a grain boundary strengthening element for strengthening austenite grain boundaries.

The boron may lower the cracking sensitivity of the steel at high temperature by strengthening the austenite grain boundary even when a small amount of boron is added. In order to improve the surface quality through the austenite grain boundary strengthening effect, it is preferable to contain 0.0005% by weight or more.

When the content of boron exceeds 0.01%, grain boundary segregation occurs at the grain boundaries of austenite, which may increase the crack sensitivity of the steel at high temperatures, thereby reducing the surface quality of the steel.

Aluminum (Al): 0.050% by weight or less (excluding 0%)

The content of aluminum is preferably limited to 0.05% by weight or less (excluding 0%).

The aluminum is added as a deoxidizer. The aluminum may react with C or N to generate a precipitate, and the hot workability may be reduced by the precipitate. Therefore, the aluminum content is preferably limited to 0.05% by weight or less (excluding 0%). The more preferable content of aluminum is 0.005 to 0.05% by weight.

S: 0.01 wt% or less (excluding 0%)

S needs to be controlled to 0.01% or less for inclusion control. If the amount of S exceeds 0.01%, the problem of hot brittleness occurs.

P: 0.03% by weight or less (except 0%)

P is an element that easily generates segregation and promotes cracking during casting. To prevent this, it should be controlled below 0.03%. If the amount of P exceeds 0.03%, castability may deteriorate, so the upper limit is made 0.03%.

Nitrogen (N): 0.1 wt% or less (excluding 0%)

In addition to carbon, nitrogen is an element that stabilizes austenite to improve toughness, and is very advantageous for enhancing strength through solid solution strengthening or precipitate formation such as carbon. However, when added in excess of 0.1%, the upper limit is preferably limited to 0.1% by weight because deterioration of physical properties or surface quality occurs due to coarsening of carbonitrides. More preferred nitrogen content is 0.001 to 0.06% by weight, even more preferred nitrogen content is 0.005 to 0.03% by weight.

The steel of the present invention contains residual iron (Fe) and other unavoidable impurities.

Unintentional impurities from the raw materials or the surrounding environment can be inevitably incorporated in a conventional steel manufacturing process and cannot be excluded.

Since these impurities are known to those skilled in the art of ordinary steel manufacturing, not all of them are specifically mentioned in the present invention.

The austenitic nonmagnetic steel having excellent hot workability according to a preferred aspect of the present invention has a sensitivity component index value of 3.4 or less represented by the following relational formula (1).

[Relationship 1]

-0.451 + 34.131 * P + 111.152 * Al-799.483 * B + 0.526 * Cr≤3.4

If the sensitivity component index value represented by the above relation (1) exceeds 3.4, there is a possibility that cracks are easily generated and propagated, thereby increasing the surface defects of the product.

The austenite-based nonmagnetic steel having excellent hot workability according to a preferred aspect of the present invention includes at least 95% of austenite in an area fraction.

Austenite, which has a low permeability as a paramagnetic material and has superior nonmagnetic properties to ferrite, is an essential microstructure for securing nonmagnetic properties.

If the area fraction of the austenite is less than 95%, it may be difficult to secure nonmagnetic properties.

The average grain size of the austenite may be 10 μm or more.

The grain size of austenite that can be implemented in the manufacturing process of the present invention is 10 μm or more, and when the grain size is greatly increased, the strength of the steel may be lowered, so that the more preferred austenite grain size is 60 μm or less.

Nonmagnetic steel having excellent hot workability according to a preferred aspect of the present invention may include one or two of precipitates and epsilon (ε) -martensite in an area fraction of 5% or less.

If one or two of precipitates and epsilon (ε) -martensite are contained in an area fraction of more than 5%, the toughness and ductility of the steel may be reduced.

EMBODIMENT OF THE INVENTION Hereinafter, the manufacturing method of the nonmagnetic steel material excellent in hot workability by this invention is demonstrated.

According to another preferred aspect of the present invention, a method for producing a non-magnetic steel having excellent hot workability includes manganese (Mn): 15 to 27 wt%, carbon (C): 0.1 to 1.1 wt%, and silicon (Si): 0.05 to 0.50 Weight%, Phosphorus (P): 0.03% or less (excluding 0%), Sulfur (S): 0.01% or less (excluding 0%), Aluminum (Al): 0.050% or less (excluding 0%), Chromium ( Cr): 5% or less (including 0%), Boron (B): 0.01% or less (including 0%), Nitrogen (N): 0.1% or less (excluding 0%), balance Fe and other unavoidable impurities Preparing a slab including and comprising a sensitivity component index value of 3.4 or less represented by the following relation (1);

[Relationship 1]

-0.451 + 34.131 * P + 111.152 * Al-799.483 * B + 0.526 * Cr≤3.4

And a cooling step of cooling the hot rolled steel.

Slab reheating stage

For hot rolling, the slab is reheated in a furnace at a temperature of 1050-1250 ° C.

At this time, if the reheating temperature is too low, less than 1050 ℃, there is a problem that the load is largely applied during rolling, alloy components are not sufficiently dissolved. On the other hand, if the reheating temperature is too high, there is a problem that the grains grow excessively and the strength is lowered, and since the reheating exceeds the solidus temperature of the steel, there is a risk of damaging the hot rollability of the steel, so the upper limit of the reheating temperature is 1250 ° C. It is preferable to limit to.

Hot rolling stage

The reheated slab is hot rolled to obtain a hot rolled steel.

The hot rolling step may include a rough rolling process and a finish rolling process.

At this time, the hot finish rolling temperature is preferably limited to 800 ~ 1050 ℃. When the hot finish rolling temperature is less than 800 ° C., the rolling load is largely applied. If the hot finish rolling temperature is more than 1050 ° C., the crystal grains grow coarse and the target strength cannot be obtained, so the upper limit is preferably limited to 1050 ° C.

Cooling stage

Cool the hot rolled steel obtained in the hot rolling step.

Cooling of the hot rolled steel after hot finishing rolling is preferably carried out at a cooling rate sufficient to suppress grain boundary carbide formation. If the cooling rate is less than 10 ℃ / s it is not enough to avoid the formation of carbides, carbides precipitate at the grain boundaries during cooling, causing ductility reduction due to premature fracture of the steel and deterioration of wear resistance is a problem, so the faster the cooling rate is advantageous If it is in the range of accelerated cooling, the upper limit of the cooling rate does not need to be particularly limited. However, in the case of normal accelerated cooling, the upper limit is preferably limited to 100 ° C / s in consideration of the fact that the cooling rate is difficult to exceed 100 ° C / s.

When cooling hot rolled steel. Cooling stop temperature is preferably limited to 600 ℃ or less. Even at a high rate of cooling, carbides may form and grow when cooling is stopped at high temperatures.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it should be noted that the following embodiments are only intended to illustrate the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

After reheating the slab satisfying the component system of Table 1 at a temperature of 1200 ℃, and hot rolled under the hot finish rolling conditions of Table 1 to produce a 12 mm thick hot rolled steel, 300 ℃ at a cooling rate of 20 ℃ / s It cooled to the temperature of.

The grain size, yield strength, tensile strength, elongation and crack sensitivity of the hot rolled steel sheet (steel) prepared as described above were measured, and the results are shown in Table 1 below.

The crack sensitivity is an element that can confirm the hot workability of the steel, as shown in Figure 2, was measured for the surface quality of the side edge, the front edge and the upper surface of the steel. The degree of sensitivity was scored for each measurement site according to the criteria of FIG. 1, and the value obtained by multiplying the scores of the three scored points as shown in Table 2 below. Table 2 has a good surface quality when the sensitivity is 3 or less.

Meanwhile, Table 2 shows the sensitivity component index values expressed as -0.451 + 34.131 * P + 111.152 * Al-799.483 * B + 0.526 * Cr.

In addition, the relationship between the sensitivity value of Table 2 and the sensitivity component index value expressed as -0.451 + 34.131 * P + 111.152 * Al-799.483 * B + 0.526 * Cr is shown in FIG.

Table 1

division	Component system (wt%)									Finish rolling temperature (℃)
division	C	Mn	Si	P	S	N	Al	B	Cr	Finish rolling temperature (℃)
Example 1	0.42	20.3	0.21	0.016	0.004	0.015	0.028	-	-	870
Example 2	0.46	25.0	0.29	0.016	0.004	0.020	0.026	0.0042	3.93	891
Example 3	0.40	19.9	0.17	0.016	0.003	0.018	0.025	0.0023	2.05	930
Example 4	0.39	21.6	0.19	0.017	0.007	0.019	0.025	0.0045	2.06	905
Example 5	0.40	25.0	0.22	0.016	0.004	0.021	0.026	-	-	885
Example 6	0.40	22.1	0.21	0.016	0.004	0.016	0.021	0.0030	-	940
Example 7	0.39	19.6	0.18	0.018	0.009	0.018	0.022	0.0038	2.03	938
Example 8	1.10	17.9	0.21	0.018	0.004	0.018	0.028	0.0040	2.70	937
Comparative Example 1	0.40	22.0	0.19	0.029	0.004	0.018	0.026	-	-	922
Comparative Example 2	0.40	22.1	0.18	0.027	0.003	0.017	0.072	0.0037	-	938
Comparative Example 3	0.40	22.2	0.20	0.015	0.004	0.017	0.051	-	-	894
Comparative Example 4	0.40	22.2	0.20	0.030	0.003	0.017	0.060	-	-	933
Comparative Example 5	0.40	22.1	0.22	0.030	0.003	0.018	0.059	-	-	885

TABLE 2

division	Surface quality		Properties
division	Component Index Value	responsiveness	Grain size (μm)	Yield strength (MPa)	Tensile Strength (MPa)	Elongation (%)
Example 1	3.21	1.00	28	371.4	977.4	50.9
Example 2	1.70	1.00	37	427.1	871.5	59.3
Example 3	2.11	1.00	32	350.6	946.0	55.9
Example 4	0.39	1.00	33	358.9	905.3	57.1
Example 5	2.98	1.50	26	360.5	918.0	27.0
Example 6	0.03	1.50	43	329.9	896.6	56.0
Example 7	0.64	1.50	29	344.1	933.7	45.9
Example 8	1.50	2.25	31	508.3	1003.9	29.5
Comparative Example 1	3.43	3.38	30	342.5	925.9	61.9
Comparative Example 2	5.52	3.38	40	325.5	887.0	53.1
Comparative Example 3	5.73	8.00	28	356.2	928.7	52.7
Comparative Example 4	7.24	8.00	35	339.0	920.0	61.4
Comparative Example 5	7.13	8.00	33	352.5	899.9	39.2

As shown in Table 1 and Table 2, Examples 1 to 8 have good surface quality with a sensitivity of 3 or less of the present invention.

Comparative Example 1 has a relatively high crack sensitivity with a high P content of 3.43.

In the case of Comparative Example 2, B was added, but since the Al content is high, the component index is decreased and thus the crack sensitivity is also reduced, but it can be seen that it is outside the scope of the present invention.

Comparative Example 3 is outside the scope of the present invention, the Al content, it can be seen that the crack sensitivity is 8.00 with a component index of 5.62.

In Comparative Examples 4 to 5, the component index was increased by the addition of P and Al, and the crack sensitivity was also increased.

On the other hand, as shown in Figure 3, when the sensitivity component index value represented by -0.451 + 34.131 * P + 111.152 * Al-799.483 * B + 0.526 * Cr is 3, 4 or less, the sensitivity is 3 or less, good surface quality It can be seen that.

Although described with reference to the embodiments above, those skilled in the art can be variously modified and changed the present invention within the scope of the basic idea of the present invention, and the scope of the invention claims It should be interpreted based on

Claims

Manganese (Mn): 15-27 wt%, Carbon (C): 0.1-1.1 wt%, Silicon (Si): 0.05-0.50 wt%, Phosphorus (P): 0.03 wt% or less (excluding 0%), Sulfur ( S): 0.01% or less (excluding 0%), Aluminum (Al): 0.050% or less (excluding 0%), Chromium (Cr): 5% or less (including 0%), Boron (B): 0.01% % Or less (including 0%), nitrogen (N): 0.1% or less (excluding 0%), balance Fe and other unavoidable impurities, and the sensitivity component index value represented by the following relation (1) is 3.4 or less,

[Relationship 1]

-0.451 + 34.131 * P + 111.152 * Al-799.483 * B + 0.526 * Cr≤3.4

(The above [P], [Al], [B] and [Cr] mean each weight percent of the corresponding element)

Non-magnetic steel with excellent hot workability, in which the microstructure contains more than 95% of austenite in an area fraction.
The nonmagnetic steel having excellent hot workability according to claim 1, wherein the average grain size of the austenite is 10 µm or more.
Manganese (Mn): 15-27 wt%, Carbon (C): 0.1-1.1 wt%, Silicon (Si): 0.05-0.50 wt%, Phosphorus (P): 0.03 wt% or less (excluding 0%), Sulfur ( S): 0.01% by weight or less (excluding 0%), aluminum (Al): 0.050% by weight (excluding 0%), chromium (Cr): 5% by weight (including 0%), boron (B): 0.01% by weight Slabs containing less than or equal to 0% (including 0%), nitrogen (N): 0.1% by weight or less (excluding 0%), residual Fe and other unavoidable impurities, and having a sensitivity component index value of 3.4 or less represented by the following relation (1): Preparing;

[Relationship 1]

-0.451 + 34.131 * P + 111.152 * Al-799.483 * B + 0.526 * Cr≤3.4

(The above [P], [Al], [B] and [Cr] mean each weight percent of the corresponding element)

A slab reheating step of reheating the slab at a temperature of 1050-1250 ° C .;

A hot rolling step of hot rolling the reheated slab to obtain a hot rolled steel; And

A method for producing a non-magnetic steel having excellent hot workability, including a cooling step of cooling a hot rolled steel.
The method of claim 3, wherein the hot finish rolling temperature during hot rolling in the hot rolling step is 800 to 1050 ° C.
The method of claim 3, wherein the cooling rate in the cooling step is 10 ~ 100 ℃ / s, characterized in that the hot workability is excellent.
The method of claim 3, wherein the cooling stop temperature during cooling in the cooling step is 600 ° C. or less. 5.
The method of claim 3, wherein the steel material has a microstructure including austenitic 95% or more of austenite in an area fraction.
The method of manufacturing a nonmagnetic steel having excellent hot workability according to claim 7, wherein the average grain size of the austenite is 10 µm or more.